[emerg] 304#0: bind() to [2002:bcdc:2142:2:927d:2b48:a642:375d]:80 failed (99: Cannot assign requested address)

This may happen at boot time if an IPv6 has *just* been assigned to an interface.  It will be in the 'tentative' state and thus nginx can't establish a listen on that IP yet.

In Debian /etc/network/interfaces, try adding this to the inet6 interface definition:

    post-up while ip -6 addr show tentative | grep . > /dev/null ; do sleep 1 ; done